Zoom synchronization
Many of us have high-quality zoom lenses for our cameras, and they work quite well. Why not take advantage of the same technology with our binoculars? The primary problem with using zoom lenses in binoculars is due to the fact that a binocular is two independent telescopes connected together. Zoom binoculars therefore must somehow maintain synchronization of the zoom mechanism (which consists of moving lens elements with each eyepiece), between both telescopes. This is accomplished by the use of a flexible metal band that passes through the ocular arms, connecting the zoom mechanism on the right side, to the zoom mechanism on the left side. Each moving part of the system has a certain amount of backlash, or "slop", that is unavoidable. Because there are a number of parts of the zoom mechanism, each adding a small amount of "slop" to the system, it is impossible to maintain perfect synchronization. Each of the two telescopes will always be leading or lagging the other, depending on which direction the zoom lever is moving. The result is two telescopes that are never exactly at the same magnification.

Collimation problems
Because zoom binoculars use moving lens elements in each eyepiece, each image will shift slightly in some direction as the lens elements move. This would not be noticeable with just one eyepiece, but because a binocular uses both eyes, collimation (the alignment between the two telescopes to each other) changes slightly (sometimes more than slightly) as the zoom lever moves from one end of the magnification range to the other. The image shift typically moves the opposite way as the direction of the zoom lever changes. The result is a binocular that is never in "perfect" alignment. As magnification increases, good alignment becomes increasingly important. Compounding this problem is the magnification synchronization problem mentioned previously. It's possible for a skilled individual to align zoom binoculars so that the extremes in variations are on equal sides of "perfect". In other words, the best-possible compromise- and "close enough" alignment is possible. But how many zoom binoculars on the market receive this level of attention before the sale? Since Oberwerk discontinued zoom binoculars, the answer is- none.

Zoom binoculars FOV (field of view)
Due to the inherent design limitations of using moving elements within a zoom eyepiece, the field of view at the low end of the magnification range is going to be severely limited compared to a fixed-power binocular. One reason people choose low-magnification binoculars is because they offer a big wide field of view- but viewing through zoom binoculars at low power is like having tunnel-vision. Compare the following FOV's between zoom and fixed-power binoculars-

10-30x60- FOV at 10x- 3.4°, 10x50 FOV- 6.0° 12-36x70- FOV at 12x- 3.2°, 12x60 FOV- 5.7° see pics --> 25-125x80- FOV at 25x- 1.25°, 25x100IF FOV- 2.5°

So zoom binoculars at the low end of the magnification range sacrifice as much as half of the field of view of fixed-power binoculars at the same magnification. In general, the higher the upper level of zoom magnification, the smaller the FOV at the low end of the range.

Marketing hype
Many manufactures push the magnification levels of their zoom binoculars to unrealistic levels in an effort to make their product appear to be the most "powerful", therefore "the best". Some zoom binoculars zoom to impossibly-high magnifications- some even over 100x! Not only will the image quality be very much degraded at these levels, good alignment is hopeless. In general, a zoom binocular that has upper magnification greater than half of the objective diameter in millimeters (eg. the "60" in 10-30x60), is more of a novelty (toy) than a usable optical instrument. Some manufactures go so far as to call these "military binoculars", which is deceptive marketing hype, to say the least. Here's an example of a 20-140x70 "Military Binocular". No military would ever use zoom binoculars, let alone models that zoom to such ridiculous magnifications. The hype doesn't stop there- this importer also claims that "only one factory possesses the technology to manufacture binoculars that can zoom up to 140 times closer!" This is probably the same factory that possesses the technology to double your gas mileage and horsepower by clipping a magic device to your fuel line ;) Not only that, "the 70mm objectives gather 70 times the light". Huh??

Summary
In general, zoom binoculars have a host of problems that can't easily be overcome, and will always be out-performed by fixed-power binoculars (of similar price). At low power, the view is almost half the size of a fixed-power binocular. At high power, image quality is degraded and most likely poorly-merged. Consider also that as magnification increases, hand-holdability decreases. The highest magnification that can be used without a mount is perhaps 15x, possibly as high as 20x for some. Beyond that, a mount of some kind will be required to obtain a usable view. So if hand-holdability AND high-magnification is desired, choose a good fixed-power binocular, such as a 15x70, or 20x80LW. Fixed-power binoculars such as these will offer as much magnification as can be held, and will have better image quality, and with none of the problems unique to zoom binoculars. If variable magnification is required, choose a binocular that has the ability to use different eyepieces to obtain different levels of magnification, such as the 25/40x100 or BT-100. These offer the best of both worlds- the versatility of multiple magnifications, but with none of the performance limitations of zoom binoculars.

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